Ferromagnetic IV group based semiconductor, ferromagnetic III-V group based compound semiconductor, or ferromagnetic II-IV group based compound semiconductor, and method for adjusting their ferromagnetic characteristics

ABSTRACT

Disclosed is a ferromagnetic group IV-based semiconductor or a ferromagnetic group III-V-based or group II-VI-based compound semiconductor, comprising a group IV-based semiconductor or a group III-V-based or group II-VI-based compound semiconductor, which contains at least one rare-earth metal element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu. The ferromagnetic characteristic of the ferromagnetic semiconductor is controlled by adjusting the concentration of the rare-earth metal element, combining two or more of the rare-earth metal elements or adding a p-type or n-type dopant. The present invention can provide a ferromagnetic group IV-based semiconductor or a ferromagnetic group III-V-based or group II-VI-based compound semiconductor which exhibits light transparency and stable ferromagnetic characteristics.

TECHNICAL FIELD

The present invention relates to a single-crystal ferromagnetic groupIV-based semiconductor or a single-crystal ferromagnetic groupIII-V-based or II-VI-based compound semiconductor, which hasferromagnetic properties achieved by adding at least one rare-earthmetal element selected from the group consisting of Ce, Pr, Nd, Pm, Sm,Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu, to a group IV-based semiconductoror a group III-V-based or group II-VI-based compound semiconductor,which is transparent to light having a wavelength in the range of theinfrared to ultraviolet regions, so as to form a mixed crystal of them.The present invention also relates to a method for adjusting aferromagnetic characteristic of the ferromagnetic semiconductor.

BACKGROUND ART

The realization of a single-crystal ferromagnetic thin film having bothlight transparency and ferromagnetic properties will open the way forobtaining optical isolators or optically assisted high-density magneticrecording essential for mass communication, and preparingelectromagnetic materials required for future mass communication. Thus,it is desired to provide a material having both light transparency andferromagnetic properties.

A group IV-based semiconductor, such as diamond, or a group III-V-basedor group II-VI-based compound semiconductor exhibits the property ofbeing transparent to light having a wavelength in the range of theinfrared to ultraviolet regions, based on their large bandgap [diamond(Eg=5.4 eV), ZnSe (Eg=2.7 eV), ZnO (Eg=3.3 eV), ZnS (Eg=3.9 eV), GaN(Eg=3.3 eV), AlN (Eg=6.4 eV), BN (Eg=6.4 eV)], and has a high excitonbinding energy. Thus, if ferromagnetic properties can be given to thesesemiconductors, the obtained ferromagnetic semiconductor will lead tomajor progress in an optical device for spin electronics, such as anoptical quantum computer utilizing coherent spin states.

The inventors previously filed a patent application for inventionconcerning a transition metal-containing ferromagnetic ZnO-basedcompound and a method of adjusting ferromagnetic characteristics thereof(Japanese Patent Laid-Open Publication No. 2001-130915).

However, neither ferromagnetic states achieved in a group III-V-based orgroup II-VI-based compound semiconductor doped with rare-earth metal,nor ferromagnetic states achieved in a group III-V-based or groupII-VI-based compound semiconductor having a high ferromagnetictransition temperature (Curie point) has been reported. Further, in thefield of silicon technologies, any silicon exhibiting ferromagneticproperties has not been reported. The realization of a silicon materialhaving ferromagnetic properties will also provide a wider applicationrange thereof.

DISCLOSURE OF INVENTION

As mentioned above, if stable ferromagnetic properties are obtained in agroup IV-based semiconductor or a group III-V-based or group II-VI-basedcompound semiconductor, the obtained ferromagnetic semiconductor can beused in combination with a light-emitting element, such as asemiconductor laser, comprising a group IV-based semiconductor or agroup III-V-based or group II-VI-based compound semiconductor having ahigh exciton binding energy, and allows for a significantly extendedapplication range of magnetooptic spin-based electronic devicesutilizing magnetooptic effects.

Further, in cases where the obtained ferromagnetic semiconductor is usedin a ferromagnetic memory utilizing the change in magnetization state tobe induced by light irradiation, the ferromagnetic semiconductor has tobe prepared to have desired ferromagnetic characteristics, for example,a ferromagnetic transition temperature (Curie temperature) adjusted at avalue (slightly greater than a room temperature) which allows themagnetization state to be changed by light irradiation.

In view of the above circumstances, it is an object of the presentinvention to provide a ferromagnetic group IV-based semiconductor or aferromagnetic group III-V-based or group II-VI-based compoundsemiconductor to be obtained using a group IV-based semiconductor or agroup III-V-based or group II-VI-based compound semiconductor which istransparent to light.

It is another object of the present invention to provide a method foradjusting a erromagnetic characteristic, e.g. ferromagnetic transitiontemperature, of a ferromagnetic group IV-based semiconductor or aferromagnetic group III-V-based or group II-VI-based compoundsemiconductor, in the process of preparing the ferromagnetic groupIV-based semiconductor or the ferromagnetic group III-V-based or groupII-VI-based compound semiconductor.

The inventors have been dedicated to researches for obtaining a singlecrystal with ferromagnetic properties using a group IV-basedsemiconductor or a group III-V-based or group II-VI-based compoundsemiconductor which has a wide bandgap suitable, particularly, as amaterial transparent to light.

During the course of these researches, the inventors found that even ifabout 1 to 25 at % of metal ion, such as Si in a group IV-basedsemiconductor, Ga in a group III-V-based compound semiconductor or Zn ina group II-VI-based compound semiconductor, is substituted with at leastone rare-earth metal element selected from the group consisting of Ce,Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu (to form a mixedcrystal of them), at a low temperature through a nonequilibrium crystalgrowth process, a single crystal can be adequately obtained.

The inventors also found that when at least one rare-earth metal elementselected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th,Dy, Ho, Er, Tm, Yb and Lu is added to a group IV-based semiconductor ora group III-V-based or group II-VI-based compound semiconductor to forma mixed crystal of them, a hole or electron is doped into thesemiconductor (the number of electrons is increased or reduced) due tothe change in electronic state, so as to allow the semiconductor to haveferromagnetic properties.

Further, the inventors found that while a group III-V-based compoundsemiconductor exhibits no ferromagnetic property by means of theformation of a mixed crystal of Gd added thereto by itself, desiredferromagnetic properties can be obtained therein by co-doping a donor,such as oxygen.

Furthermore, the inventors found that the formation of a mixed crystalby means of adding at least one rare-earth metal element selected fromthe group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm,Yb and Lu to a group IV-based semiconductor or a group III-V-based orgroup II-VI-based compound semiconductor provides the same effects asthose to be obtained by adding a hole to 4f electrons.

As above, at least one rare-earth metal element selected from the groupconsisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lucan be simply added to a group IV-based semiconductor or a groupIII-V-based or group II-VI-based compound semiconductor to form a mixedcrystal of them, so as to allow the semiconductor to be put into astable ferromagnetic state.

Through the subsequent continuous researches, the inventors found thateach of the rare-earth metals consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd,Th, Dy, Ho, Er, Tm, Yb and Lu is put into a high spin state having anelectronic spin s=1/2, 1, 3/2, 2, 5/2, 3 or 7/2, and a ferromagnetictransition temperature of a ferromagnetic semiconductor to be obtainedcan be adjusted by changing a combination of or the ratio between two ormore of these elements, or adding n-type and/or p-type dopants.

The inventors also found that the above technique can be used to providea more stabilized ferromagnetic state than antiferromagnetic andparamagnetic states, and adjust energy in the ferromagnetic state (forexample, an energy for allowing the ferromagnetic state to be generallymaintained although a slight difference in the energy causes a spinglass state analogous to an antiferromagnetic state).

Further, the inventors found that the lowest transmission wavelength ofthe obtained ferromagnetic semiconductor is varied depending on the kindof the aforementioned rare-earth metal elements, and thereby one or moreof these elements can be selectively formed as a mixed crystal to allowthe obtained ferromagnetic semiconductor to have a desired filterfunction.

Thus, the concentration and/or mixing ratio of the rare-earth metalelements can be adjusted to provide a single-crystal ferromagnetic groupIV-based semiconductor or a single-crystal ferromagnetic groupIII-V-based or group II-VI-based compound semiconductor, which hasdesired magnetic characteristics.

Specifically, the present invention provides a ferromagnetic groupIV-based semiconductor or a ferromagnetic group III-V-based or groupII-VI-based compound semiconductor, comprising a group IV-basedsemiconductor or a group III-V-based or group II-VI-based compoundsemiconductor, which contains at least one rare-earth metal elementelected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy,Ho, Er, Tm, Yb and Lu.

The group IV-based semiconductor herein is Si, diamond or Ge. The groupIII-V-based compound semiconductor is a compound, such as GaAs, GaSb,GaP, GaN, AlN, InN or BN, which is a combination of a group III atom ofB, Al, Ga, In or TI, and a group V atom of N, P, As, Sb or Bi. The groupII-VI-based compound semiconductor is a compound, such as ZnSe, ZnS,ZnTe, ZnO, CdS or CdSe, which is a combination of a group II atom of Be,Mg, Zn, Cd, Hg, Ca, Sr or Ba, and a group IV atom of O, S, Se or Te.

Each of the aforementioned rare-earth metal elements has an ionic radiusrelatively close to that of Zn, Cd, Ga, Al or In. Thus, even if theserare-earth metal elements are added to the semiconductor as a solidsolution in an amount of 1 at % to 25 at % at a low temperature througha nonequilibrium crystal growth process, a single-crystal structure ofthe semiconductor serving as a matrix can be maintained, and the matrixsemiconductor can exhibit ferromagnetic properties while maintaining thetransparency thereof.

Each of the above rare-earth metal elements selected from the groupconsisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Luis different in the atomic level of their 4f atom and the amplitude ofp-f hybridization. Thus, when at least two or more of these rare-earthmetal elements are contained in the above semiconductor, theferromagnetic characteristics of the semiconductor can be more directlyvaried as compared with hole-doping or electron-doping, so as to adjusta ferromagnetic characteristic, such as ferromagnetic transitiontemperature.

While the doping of at least one of an n-type dopant and a p-type dopantdoes not provide a direct effect as that between the rare-earth metalelements because it is incorporated into the matrix of the groupIV-based semiconductor or the group III-V-based or group II-VI-basedcompound semiconductor, the dopant can act on a 4f electron adjacent toan atom constituting the group IV-based semiconductor or groupIII-V-based or group II-VI-based compound semiconductor so as to changethe state of hole or electron to adjust the ferromagneticcharacteristic.

The present invention also provides a method of adjusting aferromagnetic characteristic of a ferromagnetic group IV-basedsemiconductor or a ferromagnetic group III-V-based or group II-VI-basedcompound semiconductor, which comprises adding either one of:

(1) at least two rare-earth metal elements selected from the groupconsisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu;

(2) the at least two rare-earth metal elements, and at least one metalelement selected from the group consisting of Th, Pa, U, Np, Pu, Am, Cm,Bk, Cf, Es, Fm, Md, No and Lr; and

(3) the above (1) or (2), and at least one of an n-type dopant and ap-type dopant, to a group IV-based semiconductor or a group III-V-basedor group II-VI-based compound semiconductor so as to adjust theferromagnetic characteristic according to a combination of therare-earth metal elements.

Further, the present invention provides a method of adjusting aferromagnetic characteristic of a ferromagnetic group IV-basedsemiconductor or a ferromagnetic group III-V-based or group II-VI-basedcompound semiconductor, which comprises adding either one of:

(1) at least one rare-earth metal element selected from the groupconsisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu;

(2) the at least one rare-earth metal element, and at least one metalelement selected from the group consisting of Th, Pa, U, Np, Pu, Am, Cm,Bk, Cf, Es, Fm, Md, No and Lr; and

(3) the above (1) or (2), and at least one of an n-type dopant and ap-type dopant, to a group IV-based semiconductor or a group III-V-basedor group II-VI-based compound semiconductor, and controlling theconcentration of one of the at least one rare-earth metal element, theat least one metal element selected from the group consisting of Th, Pa,U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No and Lr, and the at least oneof an n-type dopant and a p-type dopant so as to adjust theferromagnetic characteristic.

Specifically, according to the above methods, a ferromagnetic transitiontemperature as one of ferromagnetic characteristics can be adjusted.

In the above method, at least one or more of the rare-earth metalelements and at least one or more of the actinide elements described inthe (2) may be added to form a mixed crystal so as to adjust an energyin a ferromagnetic state, and allow the energy to be reduced as a wholeaccording to a kinetic energy of a hole or electron introduced from therare-earth metal elements by themselves, to stabilize said ferromagneticstate.

Further, at least one or more of the rare-earth metal elements and atleast one or more of the actinide elements described in the (2) may beadded to form a mixed crystal so as to control the magnitude and thepositive/negative sign of the magnetic interaction between therare-earth metal atoms according to a hole or electron introduced fromthe rare-earth metal elements by themselves, to stabilize saidferromagnetic state.

Furthermore, at least one or more of the rare-earth metal elements andat least one or more of the actinide elements described in the (2) maybe added to form a mixed crystal so as to control the magnitude and thepositive/negative sign of the magnetic interaction between therare-earth metal atoms and a light transmission characteristic to beobtained from the mixed crystallization of the rare-earth metal elementsand the actinide elements, according to a hole or electron introducedfrom the rare-earth metal elements by themselves, to provide a desiredlight filter characteristic in the ferromagnetic group IV basedsemiconductor the ferromagnetic group III-V-based or the groupII-VI-based compound semiconductor.

The rare-earth metal-containing magnetic semiconductor of the presentinvention can emit sharp light reflecting the magnetic states withoutvariation in temperature, and the emission may be utilized to generatecircularly polarized light.

In observation of a temperature dependence of the wavelength of emissioninduced by excitation using ultraviolet light, the emission wavelengthis not varied at all in the range of liquid helium temperatures to roomtemperatures. The no-variation with the temperature would result from asharp energy level in the ground state and in the excited state ofelectrons in the 4-f shell, and negligible influences of latticevibrations and a humidity dependence of the matrix semiconductor. Inconventional optical communications using emission from an AlGaSblight-emitting element, it is required to use a Peltier element or thelike because of occurrence of variation in wavelength due to temperaturechange. The semiconductor of the present invention capable of generatinglight emission without any variation in wavelength due to temperaturechange is suitable for high-performance optical communications.

The 4f-electron state of a rare-earth metal ion belonging to lanthanoidelements except for La having no 4f electron has a large electronic spinin a high spin state, and an orbital angular momentum spin based onorbital angular momentum. Further, there is 1 eV (equivalent to 8000 K)of strong ferromagnetic spin interaction between 4f electrons and 5delectrons which are valence electrons. Thus, such a rare-earth metal canbe added to the semiconductor as a solid solution to stabilize aferromagnetic state, in which an impurity band formed in the bandgap ofthe semiconductor and partly occupied by electrons to have a narrowbandwidth and a large electron correlation energy is utilized to achievea ferromagnetic state based on the gain of band energy.

FIG. 1 shows the total density of states and a partial density of statesat 5d in a case where 5 at % of Gd is doped in Si. As seen in FIG. 1, inthe total density of states with the up spin ⇑ and the down spin ↓, asharp peak is generated in the 4f-electron state, and a large exchangesplitting occurs between the up spin ⇑ and the down spin ↓ to achieve aferromagnetic state having a large magnetic moment.

FIG. 2 shows the total density of states and a partial density of statesat 5d in a case where 5 at % of Eu is doped in Si. As seen in FIG. 2, inthe total density of states with the up spin ⇑ and the down spin ↓, asharp peak is generated in the 4f-electron state, and a large exchangesplitting occurs between the up spin ⇑ and the down spin ↓ to achieve aferromagnetic state having a large magnetic moment.

FIG. 3 shows the total density of states and a partial density of statesat 5d in a case where 5 at % of Ce is doped in Si. As seen in FIG. 3, inthe total density of states with the up spin ⇑ and the down spin ↓, asharp peak is generated in the 4f-electron state, and a large exchangesplitting occurs between the up spin ⇑ and the down spin ↓ to achieve aferromagnetic state having a large magnetic moment.

According to the present invention, the kind and/or concentration of therare-earth elements can be adjusted to obtain a transparentferromagnetic semiconductor which allows visible light to passtherethrough and has a high ferromagnetic transition temperature ofgreater than room temperatures or 400 K or more, using AlN or GaN. Thephenomenon that the circular polarization direction of light is rotatedby ferromagnetism when the light passes through a transparent materialis referred to as “Kerr effect”. A ferromagnetic semiconductor providinga larger rotation angle is more excellent in performance as a device,such as optical isolators, utilizing magnetooptic effects. In Eu-dopedor Gd+O-doped GaN or AlN, the following large Kerr rotation angle (atroom temperatures, 3 mm thickness) can be obtained in the visibleregion. GaN:Eu (10 at %) 120 degree AlN:Eu (8 at %) 105 degree GaN:Gd (5at %) + O (5 at %) 110 degree AlN:Gd (10 at %) + O (10 at %) 130 degree

For example, Eu substituted for Ga site of GaN or Al site of AIN becomesEu³⁺, and a ferromagnetic transition temperature can be largelycontrolled through such electron-doping. The value of Kerr rotationangle can also be controlled in the range of zero to about 150 degreesby adjusting the concentration of Eu impurity and the amount ofelectron-doping.

Extremely high magnetooptic effects to be obtainable through the Kerreffect have potential for providing a higher performance in memories orcomputers using magnetooptic light or optical isolators. In cases wherea transition metal is contained in the aforementioned semiconductor, thetransition between 3d electrons provides light emission having a widecontinuous energy spectrum and a wavelength to be largely variedaccording to temperature change. By contrast, in case of the rare-earthmetals, the transition occurs between 4f electrons, and thereby theresulting emission advantageously has a sharp spectrum and a wavelengthto be never varied relative to temperature, which is significantlyvaluable to future data communication utilizing spin electronics.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an electron density of states in a ferromagnetic state ofGd in Si.

FIG. 2 shows an electron density of states in a ferromagnetic state ofEu in Si.

FIG. 3 shows an electron density of states in a ferromagnetic state ofCe in Si.

FIG. 4 is a graph showing the difference ΔE between the total energy inan antiferromagnetic spin glass state and the total energy in aferromagnetic state in a case where a rare-earth metal, such as Gd, Thor Dy, is added to Si to form a mixed crystal of them.

FIG. 5 is a schematic diagram showing an MBE apparatus as an example ofan apparatus for forming a ferromagnetic silicon thin film.

FIG. 6 is a graph showing the variation in ferromagnetic transitiontemperature when the concentration of a rare-earth metal to be added toSi to form a mixed crystal of them.

FIG. 7 is a graph showing the variation in ferromagnetic transitiontemperature when the ratio between two or more of rare-earth metalelements to be formed as a mixed crystal.

FIG. 8 is an explanatory diagram showing the change in magnetic statewhen Ge is added together with n-type and p-type dopants.

FIG. 9 is a graph showing a ferromagnetic transition temperature of aferromagnetic semiconductor in Example 1.

FIG. 10 is a graph showing a ferromagnetic transition temperature of aferromagnetic semiconductor in Example 2.

FIG. 11 is a graph showing a ferromagnetic transition temperature of aferromagnetic semiconductor in Example 3.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to the drawings, a ferromagnetic group IV-basedsemiconductor or a ferromagnetic group III-V-based or group II-VI-basedcompound semiconductor of the present invention, and a method foradjusting a ferromagnetic characteristic thereof, will now be described.A ferromagnetic group IV-based semiconductor or a ferromagnetic groupIII-V-based or group II-VI-based compound semiconductor of the presentinvention comprises a group IV-based semiconductor or a groupIII-V-based or group II-VI-based compound semiconductor, which containsat least one rare-earth metal element selected from the group consistingof Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu.

The following description will be made, mainly, in connection with acase where Si is used as IV-based semiconductor. In a process forobtaining a ferromagnetic material using Si as IV-based semiconductor, a4f electron of the rare-earth metal element, such as Gd, Th or Dy, isstrongly p-f-hybridized with the 3p electron of Si serving as a matrixsemiconductor. Thus, as seen in FIG. 4 showing the difference ΔE betweenthe total energy in an antiferromagnetic spin glass state and the totalenergy in a ferromagnetic state, the IV-based semiconductor exhibitsferromagnetic properties only by adding the rare-earth metal element byitself thereto to form a mixed crystal of them instead of inducing anantiferromagnetic spin glass state.

FIG. 4 shows an example where the mixed crystal ratio of the rare-earthmetal element to Si is 5 at %. However, even if the mixed crystal ratiois several at %, the semiconductor can exhibit ferromagnetic properties.Otherwise, even if the mixed crystal ratio is increased up to 5 at %,the crystallinity of the ferromagnetic material will not bedeteriorated. The mixed crystal ratio is preferably set in the range of1 at % to 100%, more preferably in the range of 5 at % to 25 at % toobtain desired ferromagnetic properties. The number of the rare-earthmetal elements is not limited to one, but two or more of the rare-earthmetal elements may be formed as a mixed crystal (alloyed) as describedlater.

FIG. 5 is a schematic diagram showing an MBE apparatus as an example ofan apparatus for forming a Si-based thin film containing theserare-earth metal elements. This apparatus comprises a substrate holder 4disposed in a chamber 1 capable of maintaining an extra-high vacuum ofabout 1.33×10⁻⁶ Pa, a substrate 5 made of Si, SiC, sapphire or the likeand held by the substrate holder 4, and a heater 7 for heating thesubstrate 5.

A cell 2 a receiving therein Si as a material (source) constituting acompound to be grown, a cell 2 b receiving therein at least onerare-earth metal element selected from the group consisting of Ce, Pr,Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu (while FIG. 5 shows asingle cell, two or more of the cells are provided when two or more ofthe rare-earth metals are subjected to mixed crystallization), a cell 2c receiving therein an n-type dopant, such as P, As or Sb, a cell 2 dreceiving therein a p-type dopant, such as B, Al or Ga, and an RFradical cell 3 a for generating radical Si. Solid materials of the Siand the rare-earth metal may be put in a cell in the form of a compoundof Si and rare-earth metal, and then the compound may be vaporized in anatomic state.

Each of the cells 2 a to 2 d receiving therein the respective solids(elemental substances) is provided with means (not shown) for heatingthe solid source to vaporize it in an atomic state. The radical cell 3 ais designed to activate the atomized sources using an RF(high-frequency) coil 8. Each of the Si, rare-earth metal element andn-type dopant is use after vaporizing a corresponding solid sourcehaving a purity of 99.99999%, in an atomic state. The atomic state ofthe Si or the rare-earth metal element may be obtained by irradiating acorresponding molecular gas with an electromagnetic wave in themicrowave region.

Then, a Si thin film 6 is grown at a temperature of 350 to 800° C. whilesupplying the n-type dopant of P, Sb or As at a flow volume of 1.33×10⁻⁵Pa, the p-type dopant of B, Al or Ga in an atomic state at a flow volumeof 6.65×10⁻⁵ Pa, and the rare-earth metal element, such as Gd, Th or Dy,in an atomic state at a flow volume of 1.33×10⁻⁵ Pa, all together ontothe substrate 5, to provide a Si thin film 6 formed as a mixed crystalcontaining the rare-earth metal element.

In the above example, the n-type and p-type dopants are doped. Bycontrast, in the aforementioned example of FIG. 4 and after-mentionedexamples of Tables 1 and 2, only Gd, Th or Dy is doped without anydoping of these dopants.

As seen in FIG. 4, the Si thin film formed as a mixed crystal containingGd, Th or Dy has a large difference between the energy in theanti-ferromagnetic spin glass state and the energy in ferromagneticstate, specifically, 4.08 (2.04)×13.6 meV (in the case of Gd), 5.14(2.57)×13.6 meV (in the case of Th), or 1.10 (0.55)×13.6 meV (in thecase of Dy), and exhibits ferromagnetic properties.

While the rare-earth metal element is doped in Si in the above example,it may also be doped in a group III-V-based nitride, such as GaN, toobtain a ferromagnetic single crystal, because GaN is different from Sionly in that its bandgap energy is greater than that of Si, and anygroup III-V-based nitride other than GaN has a four-coordinate structuresimilar to and difference only in bandgap energy from that of GaN.

However, in a case where 5 at % of Gd is doped in the semiconductor, adesired ferromagnetic material can be obtained only if 1 at % or more,preferably 3 at % or more of oxygen is doped as a donor to dope ann-type carrier in the semiconductor. Further, in a case where 10 at % ofGd is doped in the semiconductor, a desired ferromagnetic material canbe obtained only if 2 at % or more, preferably 5 at % or more of oxygenis doped as a donor to dope an n-type carrier in the semiconductor.

In the ferromagnetic Si of the present invention which is formed as amixed crystal containing the rare-earth metal element, while an Si atomis substituted with the rare-earth metal element, such as Gd³⁺, Tb³⁺ orDy³⁺, the ferromagnetic Si matrix can be maintained in a diamondstructure. In addition, the aforementioned rare-earth metal element,such as Gd, Th or Dy, has an electronic structure capable of increasingholes, and thereby stabilizes in a stable ferromagnetic state withoutany modification, as shown in FIG. 4. Further, as seen from theafter-mentioned Tables 1 and 2, this ferromagnetic Si has a largemagnetic moment. Specifically, a Si-based compound containing Gd, Th orDy has a magnetic moment (Bohr magneton) of 7 μB (Gd), 9 μB (Th) or 10μB (Dy). Thus, the present invention can provide a ferromagnetic magnethaving extremely strong magnetism.

The variation in magnetic characteristic to be caused by changing theconcentration of the rare-earth metal element was checked as follows. Inaddition to the aforementioned Si-based compound containing therare-earth metal element at a concentration of 25 at %, Si-basedcompounds containing the rare-earth metal element at 5, 10, 15 and 20 at% were prepared, and their magnetic moment (×9.247 J/T) andferromagnetic transition temperature (° K.) were measured. The magneticmoment and ferromagnetic transition temperature were determined bymeasuring a magnetic susceptibility using a SQUID (superconductingquantum interference device).

The measurement results are shown in Tables 1 and 2. As seen in Tables 1and 2, the ferromagnetic transition temperature is apt to be increasedas the mixed crystal ratio (the concentration) is increased, and isincreased approximately in proportion to the mixed crystal ratio. Thisrelationship is shown in FIG. 6. It is also proved that theferromagnetic interaction between spins is increased along with theincrease in concentration of the rare-earth metal element. TABLE 1 Typeof Concentration of Ferromagnetic Rare-Earth Rare-Earth MagneticTransition Metal Metal (at %) Moment (μB) Temperature (° K) Gd 5 6.98455 Tb 5 8.97 468 Dy 5 9.45 512

TABLE 2 Type of Concentration of Ferromagnetic Rare-Earth Rare-EarthMagnetic Transition Metal Metal (at %) Moment (μB) Temperature (° K) Gd25 6.85 690 Tb 25 8.87 860 Dy 25 9.40 880

As described above, the rare-earth metal element is put into a high spinstate which has an electronic spin s=7/2 and an orbital angular momentumL=0 for Gd, an electronic spin s=3 and an orbital angular momentum L=3for Th, or an electronic spin s=5/2 and an orbital angular momentum L=5for Dy. Further, as seen in Tables 1 and 2, and FIG. 6, the interactionbetween ferromagnetic spins and the ferromagnetic transition temperaturecan be controllably adjusted by changing the concentration of therare-earth metal element. From a practical standpoint, the ferromagnetictransition temperature is preferably arranged to be 300° K. or more.

The inventors also found that two or more of the rare-earth metalelements can be added to the semiconductor to form a mixed crystal ofthem, so as to adjust the state of holes or electrons, and obtain therespective magnetic characteristics of the two or more rare-earth metalelements together. For example, a combination of Lu and either one ofGd, Th or Dy was added to Si to form a mixed crystal of them. In a casewhere the conditions that the total concentration of Dy and Lu was setat 25 at %, and x was variously changed to form Dy_(0.25-x) Lu_(x)Si_(0.75), the ferromagnetic transition temperature could be largelyvaried, and set at zero ° K. when x=0.04, as shown in FIG. 7.

The ferromagnetic transition temperature can be set at desired value byselectively arranging x in the range of 0 to 0.10. In the same manner, acombination of Lu and Th may be added to Si to form Tb_(0.25-x) Lu_(x)Si_(0.75) while variously changing x. Further, various magnetic momentscan also be obtained, but not shown, by changing the mixing ratio of theabove two rare-earth metal elements.

In the above examples, two or more of the rare-earth metal elements aredoped to adjust the ferromagnetic characteristic. Alternatively, atleast one rare-earth metal element selected from the aforementionedgroup, and at least one metal element selected from the group consistingof Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No and Lr, whichbelong to actinide elements except for Ac having no 5f electron, may bedoped. In this case, the magnetic moment and/or the ferromagnetictransition temperature can also be adjusted by changing the respectiveconcentrations of the rare-earth metal element and the actinide element.

Further, an n-type dopant or a p-type dopant may be doped to change theamount of holes or electrons so as to adjust the ferromagneticcharacteristic.

In this case, the n-type or p-type dopant is incorporated into theconduction band or valence band of Si to act on the 4f electron of therare-earth metal element adjacent to the conduction or valence band.Thus, while all of added dopants do not always act on the 4f electron,at least a part of the dopants surely act on the 4f electron to changethe ferromagnetic state including the ferromagnetic transitiontemperature.

For example, the doping of an n-type dopant is equivalent to supplyingan electron. Thus, the doping of an n-type dopant in combination withthe mixed crystallization of either one of Gd, Th, Dy and Lu has thesame effect as that in the aforementioned example of adding Lu andeither one of Gd, Th and Dy.

FIG. 8 shows the relationship between ΔE and the concentration (at %) ofimpurity where Gd, which exhibits a significant change in ΔE=(energy inthe antiferromagnetic spin glass state)−(energy in the ferromagneticstate) in response to the doping of an n-type or p-type dopant (electronor hole), is added to Si to form a mixed crystal of them, and theimpurity is additionally doped therein.

As seen in FIG. 8, the ferromagnetic state is stabilized by introducinga hole, and vanished by doping an electron. Thus, the ferromagneticcharacteristic can be adjusted by doping the n-type or p-type dopant.While the rare-earth metal element, such as Gd, Th or Dy, originallyexhibits ferromagnetic properties, or does not have such a large changeΔE between the antiferromagnetic spin glass state and the ferromagneticstate, the doping of the n-type or p-type dopant allows theferromagnetic state as in FIG. 8 so as to adjust the ferromagnetictransition temperature.

Differently from the aforementioned adjustment through the mixedcrystallization of two or more of the rare-earth metal elements, in theadjustment using the n-type or p-type dopant, the magnetic moment itselfis maintained at a value determined by the rare-earth metal elementadded to Si to form a mixed crystal of them.

Either one of P, As and Sb may be used as the n-type dopant, and acompound of Si and either one of these elements may be used as amaterial to be doped. Preferably, the concentration of the donor is1×10¹⁸ cm⁻³. For example, the donor doped at a concentration of about10²⁰ to 10²¹ cm⁻³ is equivalent to the aforementioned rare-earth metalelement added at a mixed crystal ratio of 1 to 10 at %. As describedabove, either one of B, Al and Ga may be used as the p-type dopant.

The lowest transmission wavelength of light to pass through theferromagnetic material is varied depending on the kind or type of therare-earth material to be added to the group IV-based semiconductor or agroup III-V-based or group II-VI-based compound semiconductor which is awide bandgap semiconductor, and can be adjusted by adding two or more ofthe rare-earth materials to the semiconductor to form a mixed crystal ofthem so as to provide a light filter for cutting off light having awavelength less than a desired wavelength. For example, a ferromagneticgroup III-V-based nitride (GaN) transparent to light having a desiredwavelength can be obtained. The lowest wavelength of light to passthrough each of ferromagnetic nitrides prepared by adding 5 at % of eachof the aforementioned rare-earth metal elements to GaN to form a mixedcrystal of them were measured as shown in Table 3, wherein 5 at % ofoxygen is doped in the case of Gd. According to this example, aferromagnetic magnet transparent to light having a desired wavelengthcan be obtained. TABLE 3 Type of Rare- Concentration of Rare- Lowestearth metal earth metal (at %) wavelength (nm) GaN:Gd 5 420 GaN:Tb 5 380GaN:Dy 5 370

As mentioned above, according to the present invention, an energy in theferromagnetic state can be changed as a whole by a kinetic energy of ahole or electron introduced from the added rare-earth metal element bythemselves and others, and the level of the energy can be reduced byadjusting the concentration of the hole or electron, so as to stabilizethe ferromagnetic state. Thus, the hole or electron can be introduced tocontrollably change the magnitude and the positive/negative sign of themagnetic interaction between the rare-earth metal atoms so as tostabilize the ferromagnetic state.

While the MBE (Molecular Beam Epitaxy) apparatus is used in the aboveembodiment to form a thin film containing the rare-earth metal element,an MOCVD (Metalorganic Chemical Vapor Deposition) apparatus may be usedto form the same thin film. In this case, the metal material, such asGa, Al or the rare-earth metal, is introduced in the MOCVD apparatus inthe form of an organic metal compound, such as dimethyl gallium ordimethyl aluminum.

The MBE or MOCVD process can be used to form a thin film in anonequilibrium state while doping a transition metal element or the liketherein at a desired concentration. However, the thin film growth methodis not limited to these processes, but the thin film may be formedthrough any other suitable process, such as a laser abrasion processusing solid targets of solid Ga nitride, solid Al nitride and rare-earthmetal to form a thin film while applying an activated dopant onto asubstrate.

Further, when the rare-earth metal element or its oxide is used as thedoping material, an ECR (Electron Cyclotron Resonance) plasma deviceadapted to electronically excite the doping material using radio wave,laser, X-ray or electron beam, to put it into an atomic state may beused. The ECR plasma device may also be used for the n-type and p-typedopants. The ECR plasma device can be advantageously used to allow sucha material to be doped in an atomic state at a high concentration.

EXAMPLES Example 1

FIG. 9 shows a ferromagnetic transition temperature of a ferromagneticsemiconductor prepared by doping 5 at % of the rare-earth metal in AlN.As seen in FIG. 9, the obtained ferromagnetic semiconductor couldexhibit a high ferromagnetic transition temperature of greater than roomtemperatures or 300 K or more, which was adjusted by selecting the typeof the rare-earth metal, and a transparency to visible light. Further,it was experimentally proved that the ferromagnetic transitiontemperature (Tc) is proportional to the root of the concentration (C) ofthe rare-earth metal to be mixed (Tc∝√{square root over (C)}).

Example 2

FIG. 10 shows a ferromagnetic transition temperature of each of twoferromagnetic semiconductors prepared by doping 5 at % and 10 at % ofthe rare-earth metal in GaN. As seen in FIG. 10, each of the obtainedferromagnetic semiconductors could exhibit a high ferromagnetictransition temperature of greater than room temperatures or 400 K ormore, which was adjusted by selecting the type of the rare-earth metal,and a transparency to visible light.

Example 3

FIG. 11 shows the concentration of a donor and a ferromagnetictransition temperature of a ferromagnetic semiconductor prepared bydoping 5 at % of Gd in GaN while changing the concentration of oxygendoped therein as the donor. As seen in FIG. 11, while the ferromagneticsemiconductor prepared by doping only 5 at % of Gd in GaN exhibits noferromagnetic property, oxygen as the donor can be additionally doped toprovide desired the ferromagnetic properties, and the concentration ofthe donor can be changed to adjust the ferromagnetic transitiontemperature.

INDUSTRIAL APPLICABILITY

According to the present invention, at least one rare-earth metalelement selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu can be simply contained in a groupIV-based semiconductor or a group III-V-based or group II-VI-basedcompound semiconductor to obtain a ferromagnetic single crystal havinghigh magnetooptic effects and a high ferromagnetic transitiontemperature of greater than room temperatures. Thus, the obtainedferromagnetic single crystal can be combined with existing ZnO ortransparent conductive oxide (TCO) for use as an n-type or p-typetransparent electrode, or an optical fiber, and applied to an quantumcomputer or large-capacity magnetooptical recording, or tohigh-performance data communication or quantum computer as an opticalelectronic material usable in the range of the visible to ultravioletregions.

In addition, based on the intra 4 f-electron shell transition in therare-earth metal element, the ferromagnetic semiconductor of the presentinvention can emit light having a constant wavelength without anyvariation due to temperature change up to room temperatures. Thus, theferromagnetic semiconductor of the present invention can be adjusted tobe a p-type or n-type semiconductor which is applicable to a transparentferromagnetic semiconductor or a circularly polarized light-emittingdevice (spin-based semiconductor laser) which has extremely highmagnetooptic effects.

1. A ferromagnetic group IV-based semiconductor or a ferromagnetic groupIII-V-based or group II-VI-based compound semiconductor, which isprepared by adding at least one rare-earth metal element selected fromthe group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm,Yb and Lu, to a group IV-based semiconductor or a group III-V-based orgroup II-VI-based compound semiconductor, to form a mixed crystal ofthem so as to allow said semiconductor to have a ferromagnetic state. 2.The ferromagnetic group IV-based semiconductor or the ferromagneticgroup III-V-based or group II-VI-based compound semiconductor as definedin claim 1, which is doped with at least one of an n-type dopant and ap-type dopant.
 3. A ferromagnetic group III-V-based compoundsemiconductor comprising a group III-V-based compound semiconductor,which contains Gd and a donor.
 4. The ferromagnetic group III-V-basedcompound semiconductor as defined in claim 3, which is doped with atleast one of an n-type dopant and a p-type dopant.
 5. A magnetoopticspin electronic device comprising the ferromagnetic semiconductor asdefined in either one of claims 1 to 4, said device being adapted toutilize a magnetooptic effect of said ferromagnetic semiconductor.
 6. Amethod of adjusting a ferromagnetic characteristic of a ferromagneticgroup IV-based semiconductor or a ferromagnetic group III-based or groupII-VI-based compound semiconductor, comprising adding either one of: (1)at least two rare-earth metal elements selected from the groupconsisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu;(2) said at least two rare-earth metal elements, and at least one metalelement selected from the group consisting of Th, Pa, U, Np, Pu, Am, Cm,Bk, Cf, Es, Fm, Md, No and Lr; and (3) said (1) or (2), and at least oneof an n-type dopant and a p-type dopant, to a group IV-basedsemiconductor or a group III-V-based or group II-VI-based compoundsemiconductor, so as to allow said semiconductor to have a ferromagneticstate, and adjust said ferromagnetic characteristic according to acombination of said rare-earth metal elements.
 7. The method as definedin claim 6, wherein said ferromagnetic characteristic is a ferromagnetictransition temperature.
 8. The method as defined in claim 6, whichincludes adding said at least two rare-earth metal elements to saidgroup IV-based semiconductor or group III-V-based or group II-VI-basedcompound semiconductor to form a mixed crystal of them, so as to adjustan energy in a ferromagnetic state, and allow the energy to be reducedas a whole according to a kinetic energy of a hole or electronintroduced from said rare-earth metal elements by themselves, tostabilize said ferromagnetic state.
 9. The method as defined in claim 6,which includes adding said at least two rare-earth metal elements tosaid group IV-based semiconductor or group III-V-based or groupII-VI-based compound semiconductor to form a mixed crystal of them, soas to control the magnitude and the positive/negative sign of themagnetic interaction between the rare-earth metal atoms, and a lighttransmission characteristic to be obtained from said mixedcrystallization of said rare-earth metal elements, according to a holeor electron introduced from said rare-earth metal elements bythemselves, to provide a desired light filter characteristic in saidferromagnetic semiconductor.
 10. A method of adjusting a ferromagneticcharacteristic of a ferromagnetic group IV-based semiconductor or aferromagnetic group III-V-based or group II-VI-based compoundsemiconductor, comprising adding either one of: (1) at least onerare-earth metal element selected from the group consisting of Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; (2) said at least onerare-earth metal element, and at least one metal element selected fromthe group consisting of Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md,No and Lr; and (3) said (1) or (2), and at least one of an n-type dopantand a p-type dopant, to a group IV-based semiconductor or a groupIII-V-based or group II-VI-based compound semiconductor, so as to allowsaid semiconductor to have a ferromagnetic state, and control theconcentration of one of said at least one rare-earth metal element, saidat least one metal element selected from the group consisting of Th, Pa,U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No and Lr, and said at least oneof an n-type dopant and a p-type dopant, to adjust said ferromagneticcharacteristic.
 11. The method as defined in claim 10, wherein saidferromagnetic characteristic is a ferromagnetic transition temperature.12. The method as defined in claim 10, which includes: providing atleast two of said rare-earth metal elements; and adding said at leasttwo rare-earth metal elements to said group IV-based semiconductor orgroup III-V-based or group II-VI-based compound semiconductor to form amixed crystal of them, so as to adjust an energy in a ferromagneticstate, and allow the energy to be reduced as a whole according to akinetic energy of a hole or electron introduced from said rare-earthmetal elements by themselves, to stabilize said ferromagnetic state. 13.The method as defined in claim 10, which includes: providing at leasttwo of said rare-earth metal elements; and adding said at least tworare-earth metal elements to said group IV-based semiconductor or groupIII-V-based or group II-VI-based compound semiconductor to form a mixedcrystal of them, so as to control the magnitude and thepositive/negative sign of the magnetic interaction between therare-earth metal atoms and a light transmission characteristic to beobtained from said mixed crystallization of said rare-earth metalelements, according to a hole or electron introduced from saidrare-earth metal elements by themselves, to provide a desired lightfilter characteristic in said ferromagnetic semiconductor.